rename `Bytes::as_string()` to `to_string()`, `as_vector()` to `to_vector()`

[milyin]

Describe the release item

The as_ prefix supposes that the returned value still references to the original owner. E.g. KeyExpr::as_string_view is completely correct as the string view is just a refernence to value which contained in KeyExpt. On the other hand the to_ prefix supposes transferring the ownership. E.g. Config::to_string() creates new std::string object.

So normally the as_string and as_vector should be to_string and to_vector.

The proposal is:

• add methods to_string and to_vector to Bytes
• keep as_string and as_vector as the API is already stabilized, but mark them as deprecated and remove them from the documentation
• do all necessary renaming in the examples/tests